Chapter 7: How Cells Harvest Energy

• How our food becomes energy for our cells (ATP)

How to make ATP from glucose

Can be aerobic (with oxygen) or anaerobic (without oxygen

ATP Review

Requires coenzymes NADH and FADH2

Need to collect electrons from the food and use them to drive the machine that synthesizes

ATP

Redox Reactions

Redox with NAD and FAD

Aerobic RespirationHow cells harvest energy from glucose

Glucose + 6OGlucose + 6O22 6CO 6CO22 + 6H + 6H22O + Energy O + Energy

ATP + Heat

4 Steps of Aerobic respiration1.Glycolysis2.Oxidation of pyruvate3.Kreb’s cycle (aka Citric Acid Cycle)4.Electron transport chain

Glycolysis: 6 carbon glucose to two 3 carbon pyruvates

Occurs in Cytoplasm

Glycolysis• 6 carbon molecule broken into two 3 carbon molecules• Glucose has to be activated. This uses up 2 ATP

molecules, but kick-starts the whole thing off. • 4 ATPs are made during glycolysis, so a net gain of 2

ATPs

Glycolysis• Uses an electron carrier called NADH• Each NADH molecule carries 2 electrons that it

has removed from glucose• These electrons will be very important later!!!!!

Over-all Glycolysis• End products are 2 pyruvates (2 carbon

molecules), 2 ATPs, and 4 electrons stored on 2 NADH’s

Oxidation of Pyruvate(Breaking 3 carbon pyruvate into a 2

carbon molecule and CO2)

Oxidation of Pyruvate

• Pyruvate is transported into the mitochondrial matrix

Oxidation of Pyruvate• The 3 carbon pyruvate is

broken down into a 2 carbon molecule called AcetylCoA and a CO2

• The CO2 bubbles out of the mitochondria, then the cell

• The AcetylCoA will be further broken down in the next step

• 2 NADH’s are made in this step (stealing electrons from the organic pyruvate)

Summary: Oxidation of Pyruvate• Carbon backbone has been broken from 3 to 2

carbons

• CO2 has been made and released

• 2 NADH’s made (one per pyruvate)• No ATP made

Stage 3: Kreb’s Cycle (aka Citric Acid Cycle)

• Occurs in the matrix

• AcetylCoA is broken apart into CO2

• Final breakdown of the carbon backbone

Kreb’s Cycle(In mitochondrial matrix)

• The last electrons are removed from the organic molecules and put on electron carriers.

• FADH2 is an electron carrier like NADH. It can carry 2 electrons on each molecule.

Summary Kreb’s Cycle• No more carbon-carbon bonds or electrons from

glucose

• 6 NADH’s and 2 FADH2 created (16 electrons stored and carried)

• CO2 given off

• 2 ATPs made

Summary so far:• Glucose carbon-backbone is totally

demolished. Carbons have been released in CO2 molecules.

• All available electrons have been stripped from glucose and are now carried on 10 NADH’s and 2 FADH2’s

• Only net gain of 4 ATP’s

What to do with the electrons?

Electron Transport Chain• NADH and FADH2 drop off electrons on

the inner membrane proteins– This results in recycling NAD+ and FAD,

which will go back and participate in earlier events

– The electrons jump from one protein to the next creating something like an electric current

– This “electricity” runs something called the H+ pump. This protein moves H+ from the matrix into the inter-membrane space.

Electron Transport ChainThe H+ build up in the inter-membrane space and create a

big potential energy called the proton motive force

Electron Transport Chain• The H+ can move back into the matrix, but only through

a channel attached to an enzyme called ATP Synthase, This is called chemiosmosis.

• Like water running over a water-wheel, the H+ moving across the membrane powers the ATP Synthase to build ATP

• Approximately 32 ATPs are made in the ETC for every glucose

ATP Synthesis via Chemiosmosis

Summary: Electron Transport Chain•NADH and FADH2 are

used to power the proton pump•Protons (H+) are pumped from mitochondrial matrix into intermembrane space•These protons push through the ATP synthase making its “motor work”•ADP and Phosphate are put together to make ATP in the matrix

One last thing about the Electron Transport Chain

• The electrons moving through the ETC will finally bind to a oxygen with 2 H+, making water

• THIS IS THE REASON YOU BREATHE IN OXYGEN! To keep the electron chain moving

• If no oxygen, no ATP, and you die

• Cyanide blocks the last step of the electron transport chain, so electrons are blocked from oxygen

• The whole thing gets backed up and no electrons move through.

• Without “electric current”, the H+ pumps don’t build up the concentration gradient.

• The ATP synthase has no H+ to drive it.• You die within minutes without the new ATP!

Summary of Glycolysis and Aerobic Respiration

Aerobic –vs- Anaerobic RespirationIf no oxygen available, the glucose molecule can’t be broken down all the way.The results are 2 ATPs and either:

•2 Lactic acids (3 carbon molecules)

Or

•2 Ethanosl (2 carbon molecule) and 2 CO2

(Fermentation)

Lactate Fermentation• Uses glycolysis

• Makes 2 ATPs

• Recycles 2 NAD+

Occurs in the muscles when not enough oxygen available to make ATP neededHurts!

Alcohol Fermentation• Uses glycolysis

• Makes 2 ATPs

• Recycles 2 NAD+

• Makes carbonated ethanol (beer?)

We use yeast to do this when making alcohol. Lots of money & research goes done on this metabolic pathway.

Anaerobic vs Aerobic

Anaerobic fermentation Aerobic respiration

No oxygenNo mitochondria neededOnly net gain of 2 ATPs

Oxygen is requiredOccurs in cytoplasm and inside mitochondriaMakes about 36 ATPs

Begins with glucoseUses glycolysisIn cytoplasmUses coenzymesFAD and NAD